In the presence of iron and DPN or TPN extracts of Klebsiella aerogenes convert glutamine synthetase into an enzymatically inactive but immunologically reactive species. The inactivation of glutamine synthetase in extracts is inhibited by antimycin A, anaerobiosis, and sulfhydryl reactive compounds, but not by the serine protease inhibitors PMSF, TSF, or pentamidine isethionate. A nondialyzable extract component is also required for glutamine synthetase inactivation. The nondialyzable inactivating activity is heat sensitive and precipitates with (NH4)2SO4 (80 percent saturated, 0 degrees), but does not precipitate with streptomycin sulfate (3 g/100m1). Following ultra-filtration, glutamine synthetase inactivating activity is found in extract fractions of MW exceeding 50,000.